The procedure of replacing knee joints affected by osteoarthritis and other disease originated in the early 1960's. The success rate of this procedure improved tremendously in the following decades. However, there are an estimated 22,000 knee replacements revised yearly. Revision in other joint replacement surgery is common as well. There is still the need to improve the mechanical aspects and wear characteristics of the knee prosthesis, such that regardless of the age of the patient, any prosthesis will be expected to last a lifetime. The need also exists to improve the tools with which physicians can perform diagnostics on implanted devices in the general patient.
Much of the problem in designing prosthesis for the total knee arthroplasty (TKA) procedure lies in the fact that there is almost no in vivo data on the load conditions available. Much of the data that is available is derived from mathematical models or cadaver studies. A very limited amount of data is available using instrumented implants. There are concerns about the reliability of modeling data due to the inability to verify the modeling data with actual data and it is difficult to produce the forces and motions existing in a human using a cadaver. Most of the systems that have been used to collect data on living humans have been used on a small number of subjects and therefore the applicability of this data to a large population is questionable, and these systems have not been designed for use in a large number of devices over a long period of time, or to be manufactured on a commercial basis.
Complications resulting from patellofemoral resurfacing have been attributed as many as half of all revision total knee arthroplasties (TKA). Most complications are caused by errors in surgical technique, poor prosthetic design, or excessive patellofemoral loads of up to seven or eight times body weight during certain activities such as squatting. In many cases, however, poor knee kinematics and an inadequate understanding of the forces exerted on the prosthetic components play a key role in the wear, mal-alignment, or design flaws associated with the complications described below.
Patellofemoral complications are a prominent cause of failure in TKA. The most common TKA complication is patellofemoral subluxation (dislocation of the patella to either the medial or lateral side of the knee), which occurs in up to 29 percent of some series, resulting in patellofemoral pain and crepitus, component wear, failure, loosening and/or fracture. Other complications that lead to patellar component failure are malposition of the femoral, tibial or patellar components, poor implant design, patellar fracture, mal-alignment, inadequate patellar resection, avascular necrosis, and revision TKA.
Such complications induce many surgeons to avoid patellar resurfacing in patients with osteoarthritis and good remaining articular cartilage. However, several studies indicate increased patellofemoral problems without resurfacing, and secondary resurfacing after primary TKA with a failed nonresurfaced patella has proven inferior to resurfacing at the time of primary TKA.
In a 2002 review of all (212) revisions at one institution between 1997 and 2000, Sharkey et al. attributed early failures to infection (25.4%), loosening (16.9%), instability (21.2%), extensor mechanism deficiency (6.6%), avascular necrosis of the patella (4.2%), and isolated patella resurfacing (0.9%). The prevalent causes of overall failure were polyethylene wear (25%), aseptic loosening (24.1%), instability (21.2%), infection (17.5%), arthrofibrosis (14.6%), malalignment (11.8%), extensor mechanism deficiency (6.6%), patella necrosis (4.2%), periprosthetic fracture (2.8%), and the need to resurface the patella (0.9%). Malalignment was present in 11.8% of all implants requiring revision. Fehring, et al, in a survey of 440 patients, found similar results. These studies did not report patella-femoral complications individually, however revision of the patella-femoral component has been reported in as many as 50% of revisions.
Instrumentation of orthopedic implants has been performed by a few researchers on a small number of patients to measure the loads between the mating components. Most of this research has been performed on hip prosthesis instrumented with strain gages. G. Bergman has published research on hip loading using forces measured using strain gages. His instrumented prosthesis is powered using an inductive coil, and the measurements are sent to a personal computer using an RF telemetry system. The power for this system is generated using an inductive coil worn around the patient's leg during testing. Bergman reported loads from two patients. His data showed that joint loads range from 2.8 times body weight (BW) to 4.8 BW depending on walking speeds. These loads increased to 5.5 BW with jogging, and stumbling caused hip loads to go as high as 7.2 BW.
Similar research on hip implants has been performed by Davy and Kotzar. A similar hip prosthesis was implanted in two patients. Davy and Kotzar measured peak loads of 2.1–2.8 BW during gait and a maximum value of 5.5 BW during periods of instability during single leg stance.
K R Kaufman has reported on the design of an instrumented tibial component of TKA knee prosthesis. This device consists of a specially machined tibial tray. The tibial tray is hollowed out in Kaufman's research to form a diaphragm which is instrumented with strain gages. This device is used to determine the tibiofemoral loads. No reference as to power source or data collection method is given. We are unaware of any data collected or published using this device.
Taylor and Walker measured the forces in the distal femur in two subjects with an instrumented distal femoral replacement (DFR). The DFR was instrumented with strain gages and as in other devices inductive coupling was used to power the implant. The DFR is a large prosthesis which replaces the majority of the femur. The forces measured were as high as 3.6 BW for jogging. This device was capable of measuring the axial torque on the femur. Bending moments were greatest about the antero-posterior axis (varus-valgus) at 9.8 BW-cm.
Post-operative infection is a particularly serious threat occurring in total joint replacement and organ transplant procedures. In a survey of joint replacement surgeries performed at the Mayo Clinic in the years 1969–1996, deep wound infection (DWI) occurred in 2% of 16,035 primary total knee arthroplasty (TKA) and 1.3% of 23,519 total hip arthroplasty (THA) patients. Patients that develop DWI are at serious risk for loss of limb/organ or mortality. The estimated cost of treating post-surgical infections in orthopedic procedures alone is over $340,000,000 per year (Hansen 1999).
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Research using instrumented implants has been fairly limited as shown in the preceding paragraphs. The research that has been done has been performed to gain an understanding of joint loading conditions has consisted of on a statistically small population. The number of patients and data gathered is given to illustrate the variation in data and statistically small population. The following paragraphs will discuss recent patents in this field.
U.S. Pat. No. 6,706,005 to Roy et al., issued on Mar. 16, 2004, teaches a device for measuring only loads incident on the device using microcantilevers. No teaching of load derivation, temperature measurement, or infection sensing is present.
U.S. Pat. No. 5,470,354 with inventors Hershberger; Troy W. (Warsaw, Ind.); Booth, Jr.; Robert E. assigned to Biomet, Inc. describes a device and a method for determining proper alignment and placement of implant components during joint reconstruction surgery. This patent is primarily aimed for use in the knee joint. Provisional components are often used in joint replacement surgery to determine the proper sizes and relationships of the final components used in joint arthroplasty. In this patent, provisional components are instrumented with force transducers. These transducers are used to determine the forces in the joint and to aid in determining the proper size component and how to balance the forces in the joint. Force transducers are connected to computers and provide readings of the location and magnitude of the forces generated in the joint when the joint is moved through its range of motion during surgery. Therefore, this system is utilized for Intraoperative assessment but is not of value in assessing in vivo, weight-bearing loads after total knee implantation.
U.S. Pat. Nos. 5,360,016 and 5,197,488 invented by Nebojsa Kovacevic is assigned to NK Biotechnical Engineering Company. This patent describes a force transducer for a joint prosthesis. The transducer in this patent is similar to the one described in the research paper published by Kaufman and Kovacevic in 1996. The transducer in this paper is a highly modified tibial tray TKA component. A series of cavities are machined on the superior side of the tibial tray such that they become flexible members. Strain gages are attached to each flexure member to provide a signal corresponding to the force applied to the flexure member. No mention is made of supply power, signal conditioning, or data collection.
U.S. Pat. No. 5,425,775 issued to Kovacevic; Nebojsa (Plymouth, Minn.) and assigned to NK Biotechnical Engineering Company Nov. 2, 1993 describes a method and apparatus for measuring the forces acting on a patella includes a patella sensor comprising a sensor, a sensor cover, and multiple strain gages. This sensor consists of a significantly modified patellar insert, similar to U.S. Pat. Nos. 5,360,016 and 5,197,488 by Kovacevic for a tibial tray force transducer. This system consists of a large metal diaphragm (sensor cover) or deformable member covering the entire diameter of the patellar implant. The sensor cover is attached to the sensor and has an outer surface that is in contact with a femoral insert. The sensor cover transmits the forces acting on its outer surface to the sensor. The sensor has a plurality of strain gages mounted thereon to measure the forces acting on the sensor cover. A requirement of this sensor is a metal-backed patellar prosthesis which has been shown in multiple studies in the orthopedic literature to have inferior long term clinical results and relatively high complication rates. It also seems a requirement that a larger amount of material be resected from the patient's patella than would be with a traditional patellar insert. No mention is made of how an entire measurement system from signal conditioning to operating power is derived for this sensor. No examples of use of this device or similar devices were found in the research literature.
U.S. Pat. No. 4,822,362 Walker; Peter S. (Weston, Mass.); Ewald; Frederick C. (Weston) A prosthesis and a surgical procedure (process) therefore are provided having a relatively thin plate fitted to a resected portion of the tibial plateau with the plate fitting uniformly around a major portion of the calcareous bone of the cortical wall. A pin on the under side of the plate aligned substantially with the axis of the intramedullary canal of the tibia fixes the plate against transverse relative motion between the plate and plateau, and blades or keels also on the under side of the plate are aligned maximum density (strength) of the cancellous bone of the plateau and fix the plate against rotation relative to the plateau. The surgical procedure of the invention employs a template to assure approximate positioning, and exact interrelationship between the plate and the fixing means.
References cited and herein incorporated by reference are:    1. Sharkey P F, Hozack W J, Rothman R H, Shastri S, Jacoby S M., “Why are total knee arthroplasties failing today?”, Clin Orthop 404: 7–13, 2002.    2. Fehring T K, Odum S, Griffin G L, Mason J B, Nadaud M., “Early Failures in Total Knee Orthoplasty”, Clin Orthop 382: 315–318, 2001.    3. Brick J F, Gutmann L., “The patellofemoral component of total knee arthroplasty”, Clin Orthop 231: 163–178, 1988.    4. Bergmann G, Graichen F, Rohlmann A., “Hip Joint Loading During Walking and Running Measured in Two Patients”, Journal of Biomechanics 26 (8): 969–990, 1993.    5. Bergman G, Graichen F, Rohlmann, A, Verdonschot N, van Lenthe G H. Frictional heating of total hip implants. Part 1. measurements in patients. Journal of Biomechanics 34: 421–428, 2001.    6. Kotzar G M, Davy D T, Goldberg V M, Heiple K G, Gerilla J, Heiple K G Jr, Brown R H, Burstein A H. Telemeterized In Vivo Hip Joint Force Data: A Report on Two Patients After Total Hip Surgery. Journal of Orthopaedic Research 9 (5): 621–633, 1989.    7. Davy D T, Kotzar G M, Brown R H, Heiple K G, Goldberg V M, Heiple K G Jr., Berilla J, Burstein A H. Telemetric Force Measurements Across the Hip after Total Arthroplasty. Journal of Bone and Joint Surgery 70-A (1): 45–50, 1988.    8. Kaufman K R, Kovacevic N, Irby S E, Colwell C W. Instrumented Implant for Measuring Tibiofemoral Forces. Journal of Biomechanics. 29 (5): 667–671, 1996.    9. Taylor S J G, Walker P S. Forces and Moments Telemetered from Two Distal Femoral Replacements During Various Activities. Journal of Biomechanics 34: 839–848, 2001.    10. U.S. Pat. No. 5,470,354. Force sensing apparatus and method for orthopaedic joint reconstruction. Inventors: Hershberger; Troy W. (Warsaw, Ind.); Booth, Jr.; Robert E. Assignee: Biomet, Inc.    11. U.S. Pat. No. 5,360,016 Force transducer for a joint prosthesis. Inventor: Nebojsa Kovacevic Assignee: NK Biotechnical Engineering Company. Nov. 1, 1994.    12. U.S. Pat. No. 5,197,488 Knee joint load measuring instrument and joint prosthesis Inventor: Nebojsa Kovacevic Assignee: NK Biotechnical Engineering Company. Mar. 30, 1993.    13. U.S. Pat. No. 5,425,775. Method for measuring patellofemoral forces. Inventor: Nebojsa Kovacevic Assignee: NK Biotechnical Engineering Company. Mar. 30, 1993.    14. U.S. Pat. No. 4,822,362 Process and apparatus for tibial plateau compenent Inventors: Walker; Peter S. (Weston, Mass.) and Ewald; Frederick C. (Weston, Mass.).    15. U.S. Pat. No. 6,706,005 Apparatus and method for assessing loads on adjacent bones; issued Mar. 16, 2004 to Roy et al.